Marine propulsion systems that employ active front end (AFE) power converters and dual ac busbars are well known. With reference to FIG. 1, in a typical arrangement a series of AFE power converters 2 are used to interface ac busbars 4a, 4b to electric propulsion motors T1-T4. Each AFE power converter 2 includes a first active rectifier/inverter 6 (or ‘front end’ bridge) having ac terminals connected to the ac busbar and a second active rectifier/inverter 8 having ac terminals connected to the propulsion motor. The dc terminals of the first and second active rectifier/inverters 6, 8 are connected together by a dc link 10. Harmonic filters 12 may be connected to the ac terminals of the first active rectifier/inverter 6. In normal operation, the first active rectifier/inverter 6 will operate as an active rectifier to supply power to the dc link 10 and the second active rectifier/inverter 8 will operate as an inverter, but reverse operation may be possible in certain circumstances such as regenerative braking.
Each active rectifier/inverter 6, 8 will typically have a conventional three-phase two-level topology with a series of semiconductor power switching devices (e.g. IGBTs) fully controlled and regulated using a pulse width modulation strategy. However, in practice the active rectifier/inverters can have any suitable topology such as a three-level neutral point clamped topology or a multi-level topology, for example.
A series of prime movers (e.g. diesel engines) are connected to individual generators G1-G4 which supply ac power to the marine propulsion system through the ac busbars 4a, 4b. The ac busbars 4a, 4b may be equipped with protective switchgear with circuit breakers and associated controls. The marine propulsion system will typically include a first (or port) ac busbar 4a and a second (or starboard) ac busbar 4b. The busbars may be interconnected by a tie 14.
The ac busbars 4a, 4b may carry a low voltage (LV) supply voltage such as 690 V.
Because the marine propulsion system uses an LV supply voltage, and the fact that AFE power converters produce minimal harmonics, there is no need for converter transformers that are commonly found in other systems. This reduces the physical size and weight of the marine propulsion system. However, the use of an LV supply voltage produces high fault currents which must be accounted for in the rating of the protective switchgear. Certain operational restrictions may need to be imposed, for example the number of generators may need to be constrained to limit the maximum fault current. Although the marine propulsion system shown in FIG. 1 has advantages in terms of its physical size and weight, it also lacks some operational flexibility.